Alkaline storage battery

ABSTRACT

Disclosed is an alkaline storage battery, in which the negative electrode is constituted by a hydrogen absorbing alloy capable of absorbing/desorbing hydrogen electrochemically, and a hydrophobic material is provided in the space between the surface of the negative electrode and the separator while a hydrophilic material is provided in the inside of the negative electrode, thereby properly secure both wetting property and surface hydrophobic property of the negative electrode against the alkaline electrolytic solution. Accordingly, a hydrogen gas generated in charging the battery can be absorbed by a vapor phase reaction in the hydrophobic portion in the surface of the negative electrode which is exposed to the vapor phase and can be absorbed electrochemically in the Portion of the negative electrode which is wetted by the electrolytic solution, so that the inner pressure of the battery can be reduced to thereby make it possible to perform quick charging.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 07/646,007,filed Jan. 28, 1991 (abandoned), which is a continuation-in-part ofapplication Ser. No. 356,246, May 24, 1989, now U.S. Pat. No. 5,034,289.

BACKGROUND OF THE INVENTION

The present invention relates to an alkaline storage battery using anegative electrode of a hydrogen absorbing alloy.

Recently, hydrogen absorbing alloys capable of electrochemicallyabsorbing/desorbing a great deal of hydrogen serving as an activematerial have attracted attention as an of high energy density electrodematerial of high energy density and have been intended to be applied toa closed alkaline storage battery to be developed into a high capacitystorage battery, in particular, to be applied to a closednickel-hydrogen storage battery.

The electrode reaction in such a closed nickel-hydrogen storage batteryis as follows. ##STR1## In the reaction equation (2), M represents ahydrogen absorbing alloy.

A hydrogen absorbing alloy negative electrode for use in this typestorage battery is prepared through a process in which analkali-resisting organic high molecule, such as polyethylene,fluorocarbon polymer, or the like, is added as a binding agent to apulverized hydrogen absorbing alloy, and the resulting mixture ispressed onto or filled into an electrically conductive collector such asa punching metal or a foam metal.

When the battery is overcharged, gas generation reactions represented bythe following equations (3) and (4) occur on the positive electrode andthe negative electrode of the battery, respectively. ##STR2## Tosuppress the increase of the battery inner pressure, a method in whichan oxygen gas generated from the positive electrode according to theequation (3) is made to react with hydrogen absorbing in the negativeelectrode to thereby generate water has been employed. To suppress thegeneration of a hydrogen gas according to the equation (4), a method inwhich the capacity of the negative electrode is established to be largerhas the capacity of the positive electrode has been employed.

When the battery is charged rapidly, however, the rate of generation ofan oxygen gas is often larger than the rate of absorption of the samegas so that the oxygen gas is accumulated in the battery to therebyincrease the inner pressure of the battery. To eliminate theaforementioned disadvantage, a method of accelerating reduction of anoxygen gas by adding a noble metal catalyst such as platinum to thenegative electrode (as disclosed in Japanese Patent UnexaminedPublication No. 60-100382), a method of accelerating absorption of anoxygen gas onto the negative electrode by providing a hydrophobic layerin the hydrogen absorbing alloy negative electrode (as disclosed inJapanese Patent Unexamined Publication No. 61-118963), and the like, areknown.

However, various problems arise in the aforementioned, conventionalconstruction of the battery as follows. The method of adding a noblemetal to the negative electrode has a problem in that the material costis increased. On the other hand, the method of providing a hydrophobiclayer in the negative electrode has a problem in that the dischargingvoltage is dropped because of the uneven electrolyte distribution of thenegative electrode and the decrease of the effective surface area forthe electrochemical reaction. Further, the aforementioned method iseffective for improvement in the oxygen absorption capacity of thenegative electrode but has another problem in that the inner pressure ofthe battery is increased because hydrogen is apt be generated from thenegative electrode in charging the battery with lowering of the wettingproperty for the electrolytic solution, of the inside of the hydrogenabsorbing alloy negative electrode. In particular, this fact isremarkable when the battery is charged rapidly.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is therefore to solve theaforementioned problems.

Another object of the invention is to provide a hydrogen absorbing alloynegative electrode of a battery, in which wetting property of the insideof the negative electrode against an electrolytic solution can beimproved while hydrophobic property is kept at the same time in thevicinity of the surface of the negative electrode can be kept suitably,by which not only the inner pressure of the battery can be reduced incharging the battery rapidly but also voltage drop can be prevented indischarging the battery.

To solve the aforementioned problems, according to the presentinvention, provide is a construction of an alkaline storage battery inwhich a hydrophobic material is provided at a part of or at a greaterpart of a surface layer opposite to a negative electrode of a separator,or in which a hydrophilic resin is provided in the inside of a negativeelectrode formed of a hydrogen absorbing alloy, a hydrophobic resin isprovided on the surface portion of the negative electrode and ahydrophobic agent is provided between a separator and the negativeelectrode.

In the construction, according to the present invention, a hydrogen gasis absorbed by providing the hydrophobic resin between the surface ofthe hydrogen absorbing alloy negative electrode and the separator.Further, the wetting property of the inside of the hydrogen absorbingalloy negative electrode against an electrolytic solution is improved byuse of the hydrophilic resin to make it easily to absorb hydrogenelectrically so as to suppress generation of a hydrogen gas to therebyreduce the inner pressure of the battery in charging the batteryrapidly. Further, the voltage drop in discharging the battery can beprevented by addition of the hydrophilic resin.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a nickel-hydrogen storage battery producedaccording to the present invention;

FIG. 2 is a graph view showing the relation between charge electriccapacity and battery inner pressure in the case where a charging currentof 1 CmA at 20° C. is respectively supplied to various hydrogenocclusion alloy negative electrodes different in construction;

FIG. 3 is a graph view showing the relation between the dischargeelectric capacity and the battery voltage in the case where adischarging current of 3 CmA at 20° C. is respectively supplied tovarious hydrogen absorbing alloy negative electrodes different inconstruction;

FIG. 4 is a graph view showing the relations among the quantity ofcopolymer powder of tetrafluoroethylene-hexafluoropropylen (hereinafterreferred to as "FEP") to be added, the battery inner pressure in thecase where the battery is charged by 200% with respect to the positiveelectrode capacity with a charging current of 1 CmA at 20° C., and theintermediate voltage in the case where the battery is discharged to 0.8V with a discharging current of 3 CmA at 20° C.;

FIG. 5 is a graph view showing the relations among the quantity of PVAto be added, the battery inner pressure in the case where the battery ischarged by 200% with respect to the positive electrode capacity with acharging current of 1 CmA at 20° C., and the intermediate voltage in thecase where the battery is discharged to 0.8 V with a discharging currentof 3 CmA at 20° C.;

FIG. 6 is a graph view showing the relation between the charge capacityand the battery inner pressure when the battery is charged with acharging current of 1 CmA at 20° C.; and

FIG. 7 is a graph view showing the relation among the quantity ofaddition of FEP of the separator, the battery inner pressure and theintermediate voltage when the battery is discharged.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described hereunder with respect tovarious examples. In the examples, the hydrogen absorbing alloy used forthe negative electrode was MmNi₃.55 Co₀.75 Mn₀.4 Al₀.3. Misch metal Mm(La: about 25 wt%, Ce: about 52 wt%, Nd: about 18 wt%, Pr: about 5 wt%)which was a mixture of rare-earth elements was put into an arc meltingfurnace together with other metal samples of Ni, Co, Mn and Al. Thefurnace was evacuated to obtain a vacuum state of 10⁻⁴ to 10⁻⁵ torr.Then the metal samples were heated and melted by arc discharging underreduced pressure in an atmosphere of argon gas. Heat treatment wascarried out at 1050° C. for 6 hours in a vacuum to homogenize the metalsamples. The thus obtained alloy was ground roughly and then pulverizedby a ball mill to prepare fine powder having a particle size of notlarger than 38 μm.

By use of the thus prepared hydrogen absorbing alloy powder, thefollowing, 20 kinds of hydrogen absorbing alloy negative electrodes wereprepared.

EXAMPLE 1

An aqueous solution of poly(vinyl alcohol) (hereinafter referred to as"PVA") which was a hydrophilic resin was mixed, by an amount of PVA of0.15 wt%, into the hydrogen absorbing alloy powder to form paste. Afoamed nickel porous matrix having a porosity of 95% was filled with thethus prepared paste and pressed. Then FEP resin powder was applied, byan amount of 0.8 mg/cm², onto the both surfaces of the negativeelectrode of the thus prepared foamed nickel porous matrix to therebyobtain the negative electrode containing PVA in the inside thereof andhaving mainly a hydrophobic resin disposed on the surfaces thereof.

EXAMPLE 2

Only water was added to the aforementioned hydrogen absorbing alloypowder to form paste. A foamed nickel porous matrix having a porosity of95% was filled with the thus prepared paste and pressed. Then, "FEP" wasapplied, by an amount of 0.8 mg/cm², onto the surfaces of the negativeelectrode. Thus, a hydrogen absorbing alloy negative electrode in whichsuch a hydrophobic resin was disposed on the surfaces of the negativeelectrode but no hydrophilic resin was contained in the inside of thenegative electrodes was obtained.

EXAMPLE 3

Ethyl alcohol was added to a mixture of 97 wt% hydrogen absorbing alloypowder and 3 wt% FEP to form paste. A foamed nickel porous matrix havinga porosity of 95% was filled with the thus prepared paste and pressed.Thus, a hydrogen absorbing alloy electrode having the hydrophilic resinon the surfaces and inside thereof was prepared. These were respectivelycut into AA battery size (39 mm×80 mm×0.5 mm) to prepare negativeelectrode plates having a charge/discharge capacity of 1600 mAh and aporosity of 30 vol%.

Examples 4 to 20 show the cases where negative electrodes are preparedin the same manner as the Example 1, unless otherwise specified.

EXAMPLE 4

A hydrogen absorbing alloy negative electrode constituted by theaforementioned hydrogen absorbing alloy having a mean particle diameterof 0.1 μm was prepared.

EXAMPLE 5

A hydrogen absorbing alloy negative electrode constituted by theaforementioned hydrogen absorbing alloy having a mean particle diameterof 75 μm was prepared.

EXAMPLE 6

A hydrogen absorbing alloy negative electrode formed by dipping thehydrogen absorbing alloy powder into an alkaline solution to therebyroughen the surfaces of the hydrogen absorbing alloy particles wasPrepared.

EXAMPLE 7

A hydrogen absorbing alloy negative electrode coated with polyethyleneas a hydrophobic resin was prepared.

EXAMPLE 8

A hydrogen absorbing alloy negative electrode coated withpolytetrafluoroethylene (hereinafter referred to as "M-12") having apermeability coefficient for an oxygen/hydrogen gas, of not smaller than1×10⁻⁹ cm² /sec.atm was prepared.

EXAMPLE 9

A hydrogen absorbing alloy negative electrode coated with a hydrophobicresin by dipping the alloy powder into a solution of an FEP dispersion(hereinafter referred to as "ND-1") containing a surface active agentwas prepared.

EXAMPLE 10

A hydrogen absorbing alloy negative electrode coated with polyvinylidenefluoride (hereinafter referred to as "VDF") powder as a hydrophobicresin was prepared.

EXAMPLE 11

A hydrogen absorbing alloy negative electrode coated with FEP by anamount of 0.1 mg/cm² was prepared.

EXAMPLE 12

A hydrogen absorbing alloy negative electrode coated with FEP by anamount of 2 mg/cm² was prepared.

EXAMPLE 13

A hydrogen absorbing alloy negative electrode coated with a 2 : 1(weight proportion) mixture of platinum black capable of catalyzing thedecomposition of hydrogen and FEP, by an amount of 2.4 mg/cm², wasprepared.

EXAMPLE 14

A hydrogen absorbing alloy negative electrode coated with platinum blackby an amount of 1.6 mg/cm² and then coated with FEP by an amount of 0.8mg/cm², was prepared.

EXAMPLE 15

A hydrogen absorbing alloy negative electrode coated with a 4 : 1(weight proportion) mixture of LaNi₄ Al and FEP by an amount of 4.0mg/cm², was prepared.

EXAMPLE 16

A hydrogen absorbing alloy negative electrode coated with a 1 : 1(weight proportion) mixture of acetylene black as an electricallyconductive matter and FEP, by an amount of 1.6 mg/cm², was prepared.

EXAMPLE 17

A hydrogen absorbing alloy negative electrode containing a hydrophilicresin by an amount of 1.5 wt% in the inside of the electrode, wasprepared.

EXAMPLE 18

A hydrogen absorbing alloy negative electrode formed of an electrodeplate having a porosity of 15 vol% was prepared.

EXAMPLE 19

A hydrogen absorbing alloy negative electrode prepared by the steps of:filling a foamed nickel porous matrix with paste consisting of mixtureof the hydrogen absorbing alloy powder and PVA by an amount of 0.15% byweight of the alloy powder; applying FEP on the surface of the foamednickel porous matrix; and pressing the foamed nickel porous matrix toobtain a predetermined thickness, was prepared.

EXAMPLE 20

A hydrogen absorbing alloy negative electrode coated with FEP by anamount of 0.8 mg/cm² by dipping a negative electrode plate into adispersion solution prepared by dispersing FEP powder into an aqueoussolution of 1.5 wt% PVA, was prepared.

A negative electrode 1 selected from those twenty kinds of negativeelectrodes and a nickel positive electrode 2 prepared by filling a knownfoamed nickel matrix with nickel hydroxide were inserted into a case 4acting as a negative electrode terminal while the negative electrode 1and the positive electrode 2 were wound spirally through a separator 3formed of polyamide non-woven fabric. Then an alkaline electrolyticsolution in a predetermined amount was injected into the case 4 andsealed to prepare a closed nickel-hydrogen storage battery of 1000 mAhAA size. The structure of the thus prepared battery is shown in FIG. 1,in which a safety vent 6 provided in the inner side of a positiveelectrode cap 5 is set so as to be actuated by pressure of not lowerthan 30 kg/cm² for the purpose of measuring the inner pressure of thebattery, though, in general, such a safety vent is often set so as to beactuated by pressure of 11 to 12 kg/cm². In the drawing, the referencenumeral 7 designates a sealing plate, 8 designates an insulating gasket,and 9 designates a positive electrode collector for electricallyconnecting the positive electrode 2 to the sealing plate 7. The batteryhaving a 1 mmφ through hole formed in the bottom portion of the batterycase was fixed on a fixing apparatus and the inner pressure of thebattery was measured with a pressure sensor attached to the fixingapparatus. In the measurement of the inner pressure of the battery,charging was carried out at each of various charging rates in a range ofnot larger than 2 CmA till a time when the battery had been charged to200% of the positive electrode capacity, and the inner pressure of thebattery measured at that time was defined as the battery inner pressureat that charging rate. On the other hand, a gas generated in the batterywere collected by an aquatic substitution method and the gas compositionwas analyzed by means of gas chromatography.

In test of discharging characteristic, the battery was charged to 150%of the Positive electrode capacity with a charging current of 1 CmA in acircumstance of 20° C., and then continuously discharged to 0.8 V with adischarging current of 3 CmA.

FIG. 2 shows the charge of the battery inner pressure relative to thecharged capacity in the case where each of the batteries respectivelyincluding the hydrogen absorbing alloy negative electrodes of theExamples 1, 2 and 3 was charged to 200% of the positive electrodecapacity with a charging current of 1 CmA. As shown in FIG. 2, thebattery inner pressure upon completion of charging to 2000 mAh was 3.3kg/cm² in the case of the Example 1, 4.8 kg/cm² in the case of theExample 2, and 7.0 kg/cm² in the case of the Example 3. In the case ofthe Example 1, the increase of the battery inner pressure was startedfrom the time when the battery had been charged to about 1000 mAh. Inthe cases of the Examples 2 and 3, the increase of the battery innerpressure was started when the battery had been charged to about 800 mAh.Through analyzing the gas composition generated in the battery uponcompletion of charging to 2000 mAh, the oxygen partial pressure wasmeasured to be about 1 kg/cm² substantially equally in the all the casesof the Examples 1 to 3. Accordingly, it was understood that thedifferences in battery inner pressure among the three kinds of batterieswere caused by differences in hydrogen partial pressure.

The reason is as follows.

In the nickel-hydrogen storage battery designed to have a high-capacity,for example, of 1000 mAh in AA size, as shown in the present experiment,the balance of the negative electrode capacity (1600 mAh) against thepositive electrode capacity (1000 mAh) is not so sufficient that areaction represented by the following equations (5) to (8) progresses onthe hydrogen absorbing alloy negative electrode in charging the battery.

    M+H.sub.2 O+e.sup.- →MH+OH.sup.-                    (5)

    H.sub.2 O+e.sup.- →1/2H.sub.2 +OH.sup.31            (6)

    M+1/2H.sub.2 →MH                                    (7)

    MH+1/4.sub.2 →M+1/2H.sub.2 O                        (8)

In the equations, M represents a hydrogen absorbing alloy. In short, thehydrogen absorbing reaction represented by the equation (5) and thehydrogen generation reaction (6) represented by the equation (6) occurcompetitively in a portion of the negative electrode wetted by theelectrolytic solution. Further, the reaction of consumption of an oxygengas generated from the positive electrode, represented by the equation(8), occurs in the wetted portion at the same time. On the contrary, thereaction (7) of absorbing a hydrogen gas generated according to theequation (6) in the form of a gas progresses in another portion of thenegative electrode which is not wetted by the electrolytic solution. Thehydrophobic resin FEP acts to control the area of the hydrophobicportion on the hydrogen absorbing alloy negative electrode. It isapparent from the results of the Examples 2 and 3 that addition of thehydrophobic resin to the surface of the negative electrode is moreeffective than addition thereof to the inside of the negative electrode,and that the reaction of the equation (7) occurs mainly on the surfaceof the negative electrode. Comparing those examples, the Examples 2 and3 are inferior in wetting property of the hydrogen absorbing alloynegative electrode for the electrolytic solution because of the additionof the hydrophobic resin. Accordingly, in the Examples 2 and 3, theeffective surface area in the electrochemical reaction decreases, sothat the charging current density increases to accelerate the hydrogengas generation reaction of the equation (6) to make the rising of thebattery inner pressure early and to increase the battery inner pressurerapidly. To solve this problem, PVA, which is a hydrophilic resin, wasadded to the inside of the electrode in the Example 1. As the result,the wetting property of the inside of the hydrogen absorbing alloynegative electrode particularly for the electrolytic solution wasimproved. As compared with the Examples 2 and 3, the Example 1 had anadvantage in the point as follows. The charging current density wasreduced by the increase of the effective surface area in theelectrochemical reaction, so that the hydrogen gas generation reactionof the equation (6) was suppressed to delay the rising of the batteryinner pressure to thereby reduce the battery inner pressure. For theaforementioned reason, in the Example 1 the increase of the batteryinner Pressure can be suppressed even in the case where the battery ischarged rapidly with a charging current of 1 CmA.

FIG. 3 shows discharging curves in the cases where the three kinds ofbatteries of the respective Examples 1 to 3 were discharged with adischarging current of 3 CmA under the condition of 20° C. In FIG. 3,the battery voltage at an intermediate point of the discharge capacitywhen the battery has been discharged to 0.8 V is defined as anintermediate voltage for indicating the difference among the dischargingvoltages of the batteries.

Comparing the examples, the respective discharge capacities of thebatteries are not different but the intermediate voltages are differentremarkably. In the Example 1, the intermediate voltage was 1.150 V. Ineach of the Examples 2 and 3, the intermediate voltage was 1.100 V. Inshort, the difference between the intermediate voltages of the Example 1and each of the Examples 2 and 3 was 50 mV.

The reason is as follows. In the Example 1, the wetting property of theinside of the negative electrode for the electrolytic solution wasimproved because hydrophilic resin PVA was added to the inside of thehydrogen absorbing alloy negative electrode. Accordingly, in the Example1, the effective surface area in the electrochemical reaction increasedto reduce the discharging current density as compared with the Examples2 and 3, so that the discharge intermediate voltage increased.

For the aforementioned reason, in the Example 1 the voltage drop inhigh-efficiently discharging could be prevented.

Table 1 shows the battery inner pressures in the cases where thebatteries respectively having the twenty kinds of hydrogen absorbingalloy negative electrodes of the Examples 1 to 20 were charged with acharging current of 1 CmA to 200% of the positive electrode capacity,and the intermediate voltages in the cases where the batteries werecontinuously discharged to 0.8 V with a discharging current of 3 CmA at20° C.

                  TABLE 1                                                         ______________________________________                                                   Buttery Inner                                                                          Intermediate                                                         Pressure Voltage                                                              (Kg/cm.sup.2)                                                                          (V)                                                       ______________________________________                                        Example 1    3.3        1.150                                                 Example 2    4.8        1.100                                                 Example 3    7.0        1.100                                                 Example 4    25.4       1.125                                                 Example 5    3.3        1.080                                                 Example 6    3.3        1.180                                                 Example 7    15.4       1.148                                                 Example 8    5.6        1.140                                                 Example 9    20.4       1.150                                                 Example 10   7.0        1.153                                                 Example 11   8.3        1.162                                                 Example 12   6.6        1.105                                                 Example 13   2.5        1.165                                                 Example 14   1.8        1.170                                                 Example 15   2.4        1.162                                                 Example 16   2.3        1.200                                                 Example 17   8.4        1.155                                                 Example 18   14.3       1.125                                                 Example 19   11.2       1.151                                                 Example 20   3.5        1.175                                                 ______________________________________                                    

The effect of the particle diameter of the hydrogen absorbing alloypowder to the performance of the battery in the Examples 4 and 5 wasexamined as follows. Referring to Table 1, the battery inner pressurewas increased to 25.4 kg/cm² when the mean particle diameter of thehydrogen absorbing alloy powder was 0.1 μm. The reason is that thesurface of the alloy is oxidized more easily as the mean particlediameter of the hydrogen absorbing alloy is reduced, so that thepolarity of the hydrogen absorbing alloy negative electrode increases toaccelerate generation of a hydrogen gas in charging the battery. On thecontrary, when the mean particle diameter of the hydrogen absorbingalloy increases to 75 μm as shown in the Example 5, the true electrodesurface decreases compared with the Example 1. Accordingly, theintermediate voltage in the Example 5 is reduced by 70 mV compared withthe Example 1. It is apparent from the above description that thepreferred range of the mean particle diameter of the hydrogen absorbingalloy is from 1 to 50 μm.

In the Example 6, that is to say, in the case where a negative electrodeformed by dipping hydrogen absorbing alloy particles into an alkalinesolution to roughen the surfaces of the particles was sued, the batteryinner pressure in charging the battery was not different from that inthe Example 1 but the intermediate voltage in discharging the batterywas increased by 30 mV. As the result, it is preferable that theparticles of the hydrogen absorbing alloy powder have uneven layers inthe surfaces thereof.

In the Examples 7 to 10, the hydrophobic resin added to the surface ofthe hydrogen absorbing alloy negative electrode was examined as follows.In each of the Example 7 ion which polyethylene was disporsed on thesurface of the negative electrode, the Example 8 in which M-12 having apermeability coefficient of 1×10⁻⁹ cm² sec atm for a hydrogen gas wasdisposed, the Example 9 in which ND-1 as a FEP dispersion containing asurface active agent in a solution was disporsed, and the Example 10 inwhich VDF was disposed, the battery inner pressure in charging thebattery was increased compared with the Example 1.

This is because the hydrophobic degree of the resin in each of theExamples 7 to 10 was smaller than that of FEP, so that a solid-gasinterface sufficient for absorbing of a hydrogen gas could not be formedon the hydrogen absorbing alloy negative electrode.

In the Example 8, the solid-gas interface could be formed sufficientlyon the hydrogen absorbing alloy negative electrode. However, the Example8 was inferior in permeability of the negative electrode for a hydrogengas generated by the electrochemical reaction, so that the battery innerpressure in the Example 8 increased. In the case where the hydrogenabsorbing alloy negative electrode wa coated with a hydrophobic resinhaving a small permeability coefficient for oxygen gas, the batteryinner pressure in charging the battery increased in the same manner asdescribed above. In this case, from analyzing the gas composition, itwas found that the proportion of oxygen increased compared with theExample 1. This is because the capacity of reducing an oxygen gas wasreduced since the negative electrode was inferior in permeability for anoxygen gas.

In the Example 9, the solid-gas interface could be formed sufficientlyon the hydrogen absorbing alloy negative electrode in the same manner asin the Examples 7 to 10, because the surface active agent existing inthe solvent of ND-1 was absorbed on FEP so that the negative electrodehad an insufficient capacity of absorbing a hydrogen gas.

From the point of view of the structure of the battery safety vent orfrom the point of view of the strength of the battery case, it ispreferable that the battery inner pressure in charging the battery isnot higher than 5 kg/cm². Accordingly, the conditions of preferredhydrophobic material to be disposed on the surface layer of the hydrogenabsorbing alloy negative electrode is as follows.

(1) The material is selected from fluorocarbon polymers;

(2) The permeability coefficient for an oxygen gas or 2 hydrogen gas isnot smaller than 1×10⁻⁸ cm² / sec atm at 25° C.;

(3) When dispersion is used, no surface active agent is contained in thesolvent; and

(4) The material is tetrafluoroethylene fluorocarbon polymers oftetrafluoroethylene-hexafluoropropylen copolymer resin.

In the Examples 11 and 12, the quantity of the hydrophobic resin to beadded to the surface layer of the hydrogen absorbing alloy negativeelectrode was examined as follows. In the Example 11, when the quantityof FEP (fluorinated ethylene-propylene resin) to be added was 0.1mg/cm², the battery inner pressure in charging the battery was increasedto 8.3 kg/cm². In the Example 12, when the quantity of FEP to be addedwas 2 mg/cm², the intermediate voltage in discharging the battery wasreduced to 1.105 V because FEP acts as an insulating material to therebyincrease the polarization of the hydrogen absorbing alloy negativeelectrode in discharging the battery. FIG. 4 shows the relations amongthe quantity of FEP to be added, the battery inner pressure in chargingthe battery and the intermediate voltage in discharging the battery. Itis apparent from FIG. 4 that an optimum value exists in the quantity ofFEP to be added. Accordingly, from the double viewpoint of the batteryinner pressure in charging the battery and the intermediate voltage indischarging the battery, it is preferable that the hydrophobic resin isadded to the surface layer of the hydrogen absorbing alloy negativeelectrode by an amount in the range of from 0.15 mg/cm² to 1.5 mg/cm².

In the Examples 13 and 14, the effect by addition of a material capableof catalyzing the decomposition of a hydrogen gas, to the surface of thehydrogen absorbing alloy negative electrode and the method of additionthereof were examined. In the Example 13, the battery used a hydrogenabsorbing alloy negative electrode coated with a mixture of platinumblack capable of catalyzing the decomposition of a hydrogen gas and FEPacting as a hydrophobic material. In the Example 14, the battery used ahydrogen absorbing alloy negative electrode coated with platinum blackand then coated with FEP. Referring to Table 1, in any case, the innerbattery pressure in charging the battery decreased and the intermediatevoltage in discharging the battery increased, compared with the batteryof the Example 1 using the hydrogen absorbing alloy negative electrodein which FEP is provided mainly on the surfaces thereof. This isbecause, by the addition of platinum black, the reaction of an absorbinghydrogen gas onto the hydrogen absorbing alloy electrode as representedby the equation (7) was accelerated in charging the battery and thereaction of desorbing hydrogen from the hydrogen absorbing alloy wasaccelerated in discharging the battery. Although the examples have shownthe case where platinum black was used as a material capable ofcatalyzing the decomposition of a hydrogen gas, it is a matter of coursethat the invention is not limited to the specific examples and that thematerial may be selected from the group of platinum, palladium andpalladium black. In fact, the same excellent result could be obtained byusing the aforementioned materials.

The effect by arranging the hydrogen absorbing alloy powder having ahydrogen equilibrium pressure lower than that of MmNi₃.55 Mn₀.1 Al₀.3Co₀.75, in the surface of the hydrogen absorbing alloy negativeelectrode was examined with reference to the Example 15. The hydrogenequilibrium pressure of MmNi₃.55 Mn₀.1 Al₀.3 Co₀.75 is about 0.4 kg/cm²at 20° C., whereas the hydrogen equilibrium pressure of LaNi₄ Aldisposed in the surface of the negative electrode is about 1.8×10⁻³kg/cm² at 20° C. In this example, the battery inner pressure in chargingthe battery was 2.4 kg/cm². The value of the inner pressure wasexcellent compared with the value of 3.3 kg/cm² obtained in theExample 1. This is because the hydrogen gas absorbing reaction of theequation (7) on the negative electrode is apt to progress since thehydrogen equilibrium pressure of LaNi₄ Al is lower than that of MmNi₃.55Mn₀.1 Al₀.3 Co₀.75. The effect by LaNi₄ Al was obtained both in the casewhere it was disposed in the surface of the hydrogen absorbing alloy andin the case where it was disposed in the hydrophobic layer of thenegative electrode surface. Although this embodiment has shown the casewhere LaNi₄ AI was used as a hydrogen absorbing alloy to be added to thenegative electrode surface, it is to be understood that any suitablehydrogen absorbing alloy may be used as long as the hydrogen equilibriumpressure of the alloy is lower than that of MmNi₃.55 Mn₀.1 Al₀.3 Co₀.75.

The effect by addition of the electrically conductive material to thehydrophobic layer of the hydrogen absorbing alloy negative electrode wasexamined with reference to the Example 16. In the Example 16, thebattery inner pressure in charging the battery was 2.3 kg/cm² and theintermediate voltage in discharging the battery was 1.200 V. The valuesthus obtained in the Example 16 were excellent compared with theExample 1. This is because the electron conductivity of the hydrogenabsorbing alloy negative electrode was improved by addition of theelectrically conductive material to thereby reduce the polarization ofthe hydrogen absorbing alloy negative electrode both in charging thebattery and in discharging the battery. Although the Example 16 hasshown the case where acetylene black was used as an electricallyconductive material, the same effect could be obtained in the case wherethe electrically conductive material was selected from the group ofamorphous-structure carbon such as carbon black, ketene black and thelike, or graphite having a graphitization structure and the like.Further, when expansive graphite was used, adhesion of FEP to thenegative electrode was improved, so that the charge/discharge cyclelifetime was improved.

The quantity of the hydrophilic resin to be contained in the inside ofthe hydrogen absorbing alloy negative electrode was examined as follows.The Example 17 relates to a battery using a hydrogen absorbing alloynegative electrode containing PVA, which is a hydrophilic resin, by anamount ten times the amount in the Example 1. Referring to Table 1, thedischarging characteristic was not improved though a large amount of thehydrophilic resin was added, and the battery inner pressure in chargingthe battery increased to 8.4 kg/cm². In general, the relative quantityof the hydrogen absorbing alloy power decreases as the quantity of PVAincreases. Accordingly, addition of a large quantity of PVA is notpreferable from the point of view of high energy density of the hydrogenabsorbing alloy negative electrode. On the contrary, the Example 2 inwhich no PVA is added is not preferable from the point of view ofcharging/ discharging characteristics. FIG. 5 shows the relations amongthe quantity of PVA to be added, the battery inner pressure in chargingthe battery and the intermediate voltage in discharging the battery.From the results of FIG. 5 and from the point of view of high energydensity of the hydrogen absorbing alloy negative electrode, the optimumquantity of PVA to be added is in the range of from 0.05 to 1.0% by theweight of the hydrogen absorbing alloy. Although the example has shownthe case where PVA was used as a hydrophilic material, it is a matter ofcourse that the invention is not limited to the specific example. Thesame effect could be obtained in the case where the hydrophilic materialwas selected from other alkali-resisting resins such as carboxymethylcellulose.

The porosity of the hydrogen absorbing alloy negative electrode wasexamined as follows. In the Example 18 in which the porosity of thehydrogen absorbing alloy negative electrode was established to be 15vol%, the battery inner pressure in charging the battery was 14.3kg/cm². The hydrogen gas absorption capacity of the battery of theExample 18 was reduced compared with that of the battery of the Example1 in which the porosity of the hydrogen absorbing alloy negativeelectrode was established to be 30 vol%. The reason is as follows. TheExample 18 was inferior in the wetting property of the inside of theelectrode for the electrolytic solution because the porosity of thehydrogen absorbing alloy negative electrode was no more than 15 vol%. Asthe result, the electrochemical hydrogen absorbing reaction of theequation (5) was suppressed and the hydrogen gas generation of theequation (7) was accelerated. Further, the intermediate voltage indischarging the battery was reduced compared with this Example 1,because the wetting property of the electrode was deteriorated. On thecontrary, when the porosity of the hydrogen absorbing alloy isincreased, the charging/discharging characteristics are improved.However, the increase of the porosity is not preferable from the pointof view of high energy density of the hydrogen absorbing alloy negativeelectrode and battery. Accordingly, the preferred porosity of thehydrogen absorbing alloy negative electrode is in the range of from 20to 40 vol%.

The method of addition of the hydrophobic material to the surface of thehydrogen absorbing alloy negative electrode was examined as follows. TheExample 1 relates to a battery using a negative electrode formed by thesteps of: mixing hydrogen absorbing alloy powder and an aqueous solutionof PVA to prepare paste; filling a foamed nickel porous matrix as athree-dimensional supporting matrix with the paste; pressing thesupporting matrix containing the paste; and applying FEP to the surfaceof the negative electrode. On the other hand, the Example 19 relates toa battery using a negative electrode formed by pressing the supportingmatrix after applying FEP to the surface of the supporting matrixcontaining the paste. It was apparent from Table 1 that the batteryinner pressure in charging the battery in the Example 19 was increasedto 11.2 kg/cm² compared with the Example 1. This is because FEP in theExample 19 was distributed into the inside of the hydrogen absorbingalloy negative electrode by pressing the supporting matrix, so that thehydrophilic property of the inside of the hydrogen absorbing alloynegative electrode was deteriorated and the electrochemical hydrogenabsorbing reaction of the equation (5) was suppressed to therebyaccelerate generation of a hydrogen gas in charging the battery. For thereason, the preferred method of producing a hydrogen absorbing alloynegative electrode is the method like the Example 1 comprising the stepsof: mixing hydrogen absorbing alloy powder and an aqueous solution ofPVA to prepare paste; applying the paste to a supporting matrix throughselected one of filling, injecting, and smearing and then pressing thesupport to thereby dispose the hydrophilic material in the inside of theelectrode; and applying FEP to the surface thereof by selected one ofsmearing, dipping and injecting to thereby dispose the FEP mainly in thevicinity of the surface thereof. This hydrogen absorbing alloy negativeelectrode production method can be applied to the case where the surfaceof the hydrogen absorbing alloy negative electrode contains a materialcapable of catalyzing the decomposition of a hydrogen gas, anelectrically conductive material and hydrogen absorbing alloy powderhaving a hydrogen equilibrium pressure lower than that of MmNi₃.55 Mn₀.4Al₀.3 Co₀.75. In this case, similarly, it is preferable that thesupporting matrix containing the paste consisting of hydrogen absorbingalloy powder and an aqueous solution of PVA is pressed, and thereafterthe aforementioned materials and/or mixtures of the materials and FEPare applied to the surface of the hydrogen absorbing alloy negativeelectrode through selected one of smearing, dipping and injecting.

The Example 20 relates to a battery using a negative electrode preparedby the steps of: filling a supporting matrix with paste consisting ofhydrogen absorbing alloy powder and an aqueous solution of PVA; pressingthe supporting matrix to prepare a hydrogen absorbing alloy negativeelectrode; and adding FEP to the surface of the negative electrodethrough dipping the negative electrode into a PVA aqueous solutioncontaining FEP. The charging/discharging characteristics of the batteryof the Example 20 were as follows. The battery inner pressure incharging the battery was 3.5 kg/cm² and the intermediate voltage indischarging the battery was 1.175 V. It is apparent from comparison withthe Example 1 that the discharging characteristic of the battery of theExample 20 has been improved. Further, the inner pressure of the batteryusing the negative electrode of the Example 20 was not deteriorated evenin the case where charge/discharge was repeated over 500 cycles. This isbecause FEP is fixed firmly to the vicinity of the surface of thenegative electrode by poly(vinyl alcohol). As described above, anotherpreferred method of producing a hydrogen absorbing alloy negativeelectrode according to the invention may comprises the steps of: mixinghydrogen absorbing alloy powder and an aqueous solution of PVA toprepare paste; applying the paste to a supporting matrix throughselected one of filling, dipping and injecting; pressing the supportingmatrix; and applying a mixture of a hydrophilic material and ahydrophobic material to the surface of the supporting matrix throughselected one of smearing, dipping and injecting.

It is to be understood that poly(vinyl alcohol) may be replaced by oneof other alkali-resisting resins such as carboxymethylcellulose andmethylcellulose and the same effect can be obtained by the otheralkali-resisting resins.

A further battery was prepared as follows. A mixture of FEP andpolyethylene at a weight ratio of 2 : 1 was applied to the surface ofthe hydrogen absorbing alloy negative electrode by an amount of FEP of0.8 mg/cm² Then the negative electrode was heated at 120° C. for 30minutes. The battery was prepared by using the thus prepared negativeelectrode. The inner pressure of the battery in charging the battery was3.5 kg/cm² and the intermediate voltage thereof in discharging thebattery was 1.150 V. Those obtained values were substantially the sameas those in the Example 1. However, the battery inner pressure was notdeteriorated in the same manner as in the Example 20 even in the casewhere charge/discharge was repeated over 500 cycles. This is because FEPwas fixed firmly to the surface of the negative electrode bypolyethylene. Polyethylene used herein may be replaced by one ofthermoplastic resins, such as polypropylene, poly(vinyl chloride), ABSresin and polystyrene, having a melting point lower than that of FEP. Inthe case where polyethylene was replaced by one of the aforementionedthermoplastic resins, the same effect as described above was obtained.As described above, a further preferred method of producing a hydrogenabsorbing alloy negative electrode according to the invention maycomprises the steps of: mixing hydrogen absorbing alloy powder and anaqueous solution of PVA to prepare paste; applying the paste to asupporting matrix through selected one of filling, dipping andinjecting; pressing the supporting matrix; applying a mixture of ahydrophilic material and a thermoplastic resin having a melting pointlower than that of the hydrophobic material to the surface of thesupporting matrix through selected one of smearing, dipping andinjecting; and heating the supporting matrix at a temperature at whichthe thermoplastic resin is melted but the hydrophobic material is notmelted.

With respect to the Examples 1 to 20, substantially the same resultswere obtained even in the case where the composition of the hydrogenabsorbing alloy was changed within the range represented by the generalformula A_(1-x) B_(x) C_(y). However, when MmNi₅ was used as a hydrogenabsorbing alloy having a CaCu₅ -type crystalline structure,pulverization of hydrogen absorbing alloy Particles progressed byrepeating the charge/discharge cycle, so that the particles were droppedout of the electrode supporting matrix and the discharge capacity wasreduced to deteriorate the cycle lifetime of the battery. To solve theproblem, at least one metal selected from the group consisting of Ti,Zr, Ca, Y, Hf, Co, Mn, Al, Fe, Cu and Cr was added to MmNi₅ to prepare amulticomponent alloy. The progress of pulverization of hydrogenabsorbing alloy particles by repeating the charge/discharge cycle wassuppressed by the multicomponent alloy, so that the cycle lifetimecharacteristic of the battery was improved. However, when Ti, Zr, Ca, Yor Hf was added by an amount of not smaller than 0.2 atomic ratio, whenCo or Cu was added by an amount of not smaller than 1.0 atomic ratio,when Fe or Cr was added by an amount of not smaller than 0.3 atomicratio, when Mn was added by an amount of not smaller than 0.6 atomicratio or when Al was added by an amount of not smaller than 0.5 atomicratio, the alloy phase effective for absorbing hydrogen was reduced sothat the discharge capacity was undesirably reduced. On the contrary,when Ni was added by an amount of not larger than 3.5 atomic ratio, thedischarge capacity of the hydrogen absorbing alloy negative electrodewas deteriorated in the same manner as described above. On the otherhand, when the weight ratio of the hydrogen absorbing alloy was widelychanged from CaCu₅ to CaCu₄.7 or CaCu₅.3, the discharge capacity of thehydrogen absorbing alloy negative electrode was undesirably deterioratedin the same manner as described above. As described above, the preferredhydrogen absorbing alloy used in the hydrogen absorbing alloy negativeelectrode is represented by the general composition formula A_(1-x)B_(x) C_(y), in which A is selected from the group consisting of La,mixtures of La and rare-earth elements, and misch metals; B is selectedfrom the group consisting of Ti, Zr, Ca, Y, Hf and mixtures thereof; xhas a value within the range 0≦x ≦0.2; C is selected from the groupconsisting of Ni, Co, Mn, Al, Fe, Cu, Cr and mixtures thereof; and y hasa value within the range 4.7≦y ≦5.3, made up as follows, y ≧3.5 for Ni,y≦1.0 for Co, y≦0.6 for Mn, y≦0.5 for Al, y≦0.3 for Fe, y≦1.0 for Cu,and y≦0.3 for Cr.

Further, V was added to the aforementioned hydrogen absorbing alloy toprepare a hydrogen absorbing alloy represented by the formula MmNi₃.55Co₀.75 Mn₀.4 Al₀.3 V₀.02. When the negative electrode of the battery wasformed of this alloy, the battery inner pressure in charging the batterywas 2.8 gk/cm² and the intermediate voltage in discharging the batterywas 1.158 V. Thus, the battery was improved compared with the Example 1.This is because the lattice constant of the hydrogen absorbing alloy wasincreased by addition of V thereby hydrogen can diffuse rapidly in thehydrogen absorbing alloy phase. The effect by addition of V was foundwhen V was added by an amount of not larger than 0.02 atomic ratio.However, when V was added by an amount of not smaller than 0.3, thealloy phase effective for absorbing hydrogen was reduced so that thedischarge capacity was undesirably reduced. Accordingly, it ispreferable to add V by an amount in the range of from 0.02 to 0.3 atomicratio.

Further, In was added to the aforementioned hydrogen absorbing alloy toprepare a hydrogen absorbing alloy represented by the formula MnNi₃.55Co₀.75 Mn₀.4 Al₀.3 In₀.02. When a battery was prepared by using thealloy in the negative electrode thereof, the inner pressure of thebattery in charging the battery was 2.5 kg/cm². In short, the chargingcharacteristic of the battery was improved compared with the Example 1.This is because the hydrogen overvoltage of the hydrogen absorbing alloynegative electrode in charging the battery was increased to suppressgeneration of hydrogen. The effect by addition of In was found when Inwas added by an amount of not larger than 0.02 atomic ratio. However,when In was added by an amount of not smaller than 0.1, the dischargecapacity was undesirably reduced. Accordingly, it is preferable to addIn by an amount in the range of from 0.02 to 0.3 atomic ratio. The sameeffect was obtained in the case where In was replaced by Tl or Ga.

Next, description will be made about the specific examples of theinvention in which a hydrophobic agent was disposed on one surface of aseparator contacting the negative electrode together with comparativeexamples.

EXAMPLE 21

An aqueous solution of PVA was added, by an amount of PVA of 0.2 wt%, tothe above hydrogen absorbing alloy powder to form paste. A foamed nickelmatrix having a porosity of 95% was filled with the paste and pressed toa predetermined thickness to thereby form a negative electrode. On theother hand, FEP was uniformly applied, by an amount of 0.5 mg/cm², ontoone surface of non-woven woven fabric of sulfonated polypropylenearranged to be contact with the negative electrode, to thereby form aseparator. A closed nickel-hydrogen storage battery was constituted bythe negative electrode, the separator and the nickel positive electrodeas described above.

EXAMPLE 22

A closed nickel-hydrogen storage battery having the same structure asthat in Example 21 and being formed by dipping the hydrogen absorbingalloy powder into an alkaline solution to thereby roughen the surfacesof the hydrogen absorbing alloy particles was prepared.

EXAMPLE 23

A closed nickel-hydrogen storage battery using a negative electrodeformed by applying platinum black by an amount of 1 mg/cm² onto thesurface thereof in the same manner as in Example 21 and a separatorformed in the same manner as in Example 21 was prepared.

EXAMPLE 25

A closed nickel-hydrogen storage battery formed by adding platinum blackto the nickel-hydrogen storage battery prepared in Example 21, so as tobe free from electrical contact between the platinum black and thepositive and negative electrodes, was prepared.

COMPARATIVE EXAMPLE 1

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed ofsulfonated polypropylene non-woven fabric being not covered with FEP wasprepared.

COMPARATIVE EXAMPLE 2

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying polyethylene resin powder (hereinafter referred to as "PE"), by0.5 mg/cm², uniformly onto one surface of sulfonated polypropylenenon-woven fabric being contact with the negative electrode was prepared.

COMPARATIVE EXAMPLE 3

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying the above M-12 having a permeability coefficient of 1×10⁻⁹ cm²/sec.atm for oxygen gas and hydrogen gas, by an amount of 0.5 mg/cm²,onto a surface of sulfonated polypropylene non-woven fabric being incontact with the negative electrode was prepared.

COMPARATIVE EXAMPLE 4

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying an FEP dispersion (ND-1) including a surface active agent in asolution, by an amount of 0.5 mg/cm² calculated as solid FEP, onto asurface of sulfonated polypropylene non-woven fabric being in contactwith the negative electrode was prepared.

COMPARATIVE EXAMPLE 5

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying VDF, by an amount of 0.5 mg/cm², onto a surface of sulfonatedpolypropylene non-woven fabric being in contact with the negativeelectrode was prepared.

COMPARATIVE EXAMPLE 6

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying FEP, by an amount of 0.05 mg/cm², uniformly onto a surface ofsulfonated polyropylene non-woven fabric being in contact with thenegative electrode was prepared.

COMPARATIVE EXAMPLE 7

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying FEP, by an amount of 2.0 mg/cm², uniformly onto a surface ofsulfonated polypropylene non-woven fabric being in contact with thenegative electrode was prepared.

COMPARATIVE EXAMPLE 8

A closed nickel-hydrogen storage battery using a negative electrodeprepared in the same manner as in Example 21 and a separator formed byapplying FEP, by an amount of 0.5 mg/cm², uniformly onto a surface ofpolyamide non-woven fabric being in contact with the negative electrodewas prepared.

COMPARATIVE EXAMPLE 9

A closed nickel-hydrogen storage battery having the same structure as inExample b 21 except that the mean particle diameter of the hydrogenabsorbing alloy is 0.1 μm was prepared.

COMPARATIVE EXAMPLE 10

A closed nickel-hydrogen storage battery having the same structure as inExample b 21 except that the mean particle diameter of the hydrogenabsorbing alloy is 75 μm was prepared.

FIG. 6 shows the change of the battery inner pressure relative to thecharged capacity in the case where each of the closed nickel-hydrogenstorage batteries of the Example 21 and the Comparative Example 1 wascharged to 200% of the positive electrode capacity with a chargingcurrent of 1 CmA. As shown in FIG. 6, the battery inner pressure uponcompletion of charging to 2000 mAh was 5.5 kg/cm² in the case of theExample 21. However, in the case of the Comparative Example 1, thebattery inner pressure upon completion of charging to 1500 mAhapproached 10 kg/cm² and the battery inner pressure upon completion ofcharging to 2000 mAh was about 30 kg/cm².

The reasons are as follows.

In the closed nickel-hydrogen storage battery designed to have ahigh-capacity, for example, of 1000 mAh in AA size, as shown in thepresent invention, the balance of the negative electrode capacity (1600mAh) against the positive electrode capacity (1000 mAh) is not sosufficient that a reaction represented by the equations (5) to (8)competitively progresses on the hydrogen absorbing alloy negativeelectrode in charging the battery.

In short, the hydrogen absorbing reaction represented by the equation(5) and the hydrogen generation reaction represented by the equation (6)occur competitively in a portion of the negative electrode wetted by theelectrolytic solution. On the contrary, the reaction (7) of absorbing ahydrogen gas generated according to the equation (6) in the form of agas and the reaction of consumption of an oxygen gas generated from thepositive electrode, represented by the equation (8), occur in anotherportion of the negative electrode which is not wetted by theelectrolytic solution.

The battery inner pressure can be reduced by suppressing the hydrogengeneration reaction represented by the equation (6) and accelerating thehydrogen absorbing reaction represented by the equations (5) and (7) andthe oxygen consumption reaction represented by the equation (8).

In the case of the Comparative Example 1 showing a conventional battery,the amount of the electrolytic solution in the neighbor of the negativeelectrode is so large that the hydrogen absorbing reaction representedby the equation (7) and the oxygen consumption reaction represented bythe equation (8) cannot progress sufficiently. Accordingly, in the caseof the Comparative Example 1, the battery inner pressure increases. Onthe contrary, in the case of the Example 21, the battery is formed byapplying FEP as a hydrophobic resin onto the separator to optimize theelectrolytic solution distribution in the neighbor of the negativeelectrode surface being in contact with the separator, to therebyaccelerate the gas consumption reaction represented by the equations (7)and (8). Accordingly, in the case of the Example 21, the battery innerpressure is reduced. For the aforementioned reason, in the Example 21the increase of the battery inner pressure can be suppressed even in thecase where the battery is charged rapidly with a charging current of 1CmA.

Table 2 shows the battery inner pressures in the cases where thebatteries respectively having the fourteen kinds of hydrogen absorbingalloy negative electrodes of the Examples 21 to 24 and the ComparativeExamples 1 to 10 were charged with a charging current of 1 CmA to 200%of the positive electrode capacity, and the intermediate voltages in thecases where the batteries were continuously discharged to 0.8 V with adischarging current of 3 CmA at 20° C.

                  TABLE 2                                                         ______________________________________                                                     Buttery Inner                                                                          Intermediate                                                         Pressure Voltage                                                              (kg/cm.sup.2)                                                                          (V)                                                     ______________________________________                                        Example 21     5.5        1.15                                                Example 22     5.5        1.18                                                Example 23     3.5        1.15                                                Example 24     4.0        1.15                                                Comp. Example 1                                                                              30.0       1.16                                                Comp. Example 2                                                                              17.0       1.15                                                Comp. Example 3                                                                              10.0       1.14                                                Comp. Example 4                                                                              26.0       1.13                                                Comp. Example 5                                                                              9.0        1.14                                                Comp. Example 6                                                                              9.0        1.16                                                Comp. Example 7                                                                              7.0        1.13                                                Comp. Example 8                                                                              5.5        1.15                                                Comp. Example 9                                                                              25.0       1.18                                                Comp. Example 10                                                                             5.5        1.08                                                ______________________________________                                    

In the Comparative Examples 2 to 5, the hydrophobic resin added to thesurface of the separator, as to the kind of the hydrophobic resin, wasexampled as follows As shown in Table 2, in each of the ComparativeExample 2 in which polyethylene (PE) was disposed on the surface of theseparator, the Comparative Example 3 in M-12 having a permeabilitycoefficient of 1'10⁻⁹ cm² /sec atm for a hydrogen gas was disposed, theComparative Example 4 in which ND-1 as a FEP dispersion containing asurface active agent in a solution was disposed, and the ComparativeExample 5 in which VDF was disposed, the battery inner pressure incharging the battery was increased compared with the Example 21.

This is because the hydrophobic degree of the resin in each of theComparative Examples 2 and 5 was smaller than that of FEP and the amountof the electrolytic solution in the neighbor of the negative electrodesurface was larger, so that the hydrogen gas absorbing reaction and theoxygen gas consumption reaction could not progress sufficiently. In theComparative Example 3, the electrolytic solution distribution in theneighbor of the negative electrode surface was optimized but thepermeability of the negative electrode for hydrogen gas generated on thenegative electrode according to the reaction equation (6) and oxygen gasgenerated from the positive electrode was deteriorated, so that thebattery inner pressure was increased.

In the Comparative Example 4, the hydrogen gas absorbing reaction andthe oxygen gas consumption reaction could not progress sufficiently,because the surface active agent existing in the solvent of ND-1 wasabsorbed on FEP so that the amount of the electrolytic solution in theneighbor of the negative electrode was excessive as in the ComparativeExamples 2 and 5.

From the point of view of the structure of the battery safety vent orfrom the point of view of the strength of the battery case, it ispreferable that the battery inner pressure in charging the battery isnot higher than 7 or 8 kg/cm². Accordingly, the conditions of preferredhydrophobic resin material to be disposed on one surface of theseparator being in contact with the negative electrode are as follows.

(1) The material is selected from fluorocarbon polymers;

(2) The permeability coefficient for an oxygen gas or an hydrogen gas isnot smaller than 1×10⁻⁸ cm² /sec atm;

(3) When dispersion is used, no surface active agent is contained in thesolvent; and

(4) The material is polytetrafluoroethylene ortetrafluoroethylene-hexafluoropropylene copolymer resin.

In the Comparative Examples 6 and 7, the quantity of the hydrophobicresin to be added to the surface of the separator being in contact withthe negative electrode was examined as follows. In the ComparativeExample 6, when the quantity of FEP to be added was 0.05 mg/cm², thebattery inner pressure in charging the battery was increased to 9.0kg/cm². In the Comparative Example 7, when the quantity of FEP to beadded was 2.0 mg/cm², the battery inner pressure in charging was about7.0 kg/cm² but the intermediate voltage in discharging was reduced to1.13 V because FEP acts as an electrically insulating material tothereby increase the polarization in discharging.

FIG. 7 shows the relations among the quantity of FEP to be added, thebattery inner pressure in charging the battery and the intermediatevoltage in discharging the battery. It is apparent from FIG. 7 that anoptimum value exists in the quantity of FEP to be added. Accordingly,from the double viewpoint of the battery inner pressure in charging thebattery and the intermediate voltage in discharging the battery, it ispreferable that the hydrophobic resin is added to the surface of theseparator being in contact with the negative electrode, by an amount inthe range of from 0.06 mg/cm² to 1.2 mg/cm².

In the Comparative Example 8, the material used as the separator wasexamined. When the separator was constituted by polyamide non-wovenfabric, the polyamide separator had the same battery inner pressure andthe same discharging characteristic as the polypropylene separator butthe self-discharging characteristic at high temperature wasdeteriorated. This tendency did not change even in the case where wovenfabric was used as the separator.

As a result, it is preferable that the separator is selected frompolypropylene woven fabric and polypropylene non-woven fabric.

In the Comparative Examples 9 and 10, the particle size of the hydrogenabsorbing alloy was examined. In the Comparative Example 9, when themean particle diameter of the hydrogen absorbing alloy was 0.1 μm, thebattery inner pressure was increased to 25.0 kg/cm². This is because asthe mean particle diameter of the hydrogen absorbing alloy decreased,the surface of the alloy became oxidized easily, so that thepolarization of the hydrogen absorbing alloy negative electrode incharging the battery was increased to make it easy to generate hydrogengas. When the mean Particle diameter of the hydrogen absorbing alloy waslarge as 75 μm, as shown in the Comparative Example 10, the true surfacearea of the electrode was reduced compared with that in the Example 21,so that the intermediate voltage was reduced by 70 mV.

Accordingly, it is preferable that the mean particle diameter of thehydrogen absorbing alloy is in a range of 1 μm to 50 μm.

In the Example 22, when a negative electrode having a rough layer in thesurfaces of the hydrogen absorbing alloy particles formed by dipping inan alkaline solution was used, the battery inner pressure did not changecompared with the Example 21 but the intermediate voltage was increasedby 30 mV. Accordingly, it is preferable that the surfaces of thehydrogen absorbing alloy particles have a rough layer.

In the Example 23, the effect of addition of the material capable ofcatalyzing the decomposition of hydrogen gas upon the hydrogen absorbingalloy negative electrode was examined. When platinum black capable ofcatalyzing the decomposition of hydrogen gas was added to the surface ofthe hydrogen absorbing alloy negative electrode, the battery innerpressure was further reduced to 3.5 kg/cm² compared with the Example 21.This is because the vapor-phase hydrogen absorbing reaction representedby the equation (7) was accelerated in charging the battery. Anysuitable material, such as platinum, palladium, palladium black, etc.,other than platinum black may be used as the material capable ofcatalyzing the decomposition of hydrogen gas. It is a matter of coursethat the material may be added into the inside of the negativeelectrode.

In the Example 24, the effect of the material capable of catalyzing thedecomposition of hydrogen gas and oxygen gas in the case where thematerial was included in the inside of the battery in a state in whichthe material was not in electrical contact with the positive andnegative electrodes was examined. When platinum black was disposed inthe inside of the battery in a state in which it was not in contact withelectrical contact with the positive and negative electrodes, thebattery inner pressure was further reduced to 4.0 kg/cm² compared withthe Example 21. This is because the reaction of oxygen gas generated inthe positive electrode and hydrogen gas generated in the negativeelectrode to produce water was accelerated on platinum black. Othermaterials, such as platinum, palladium, etc., capable of catalyzing thedecomposition of hydrogen gas and oxygen gas may be used to attain thesame effect.

The phenomenon in the Examples 21 to 24 did not change even in the casewhere a negative electrode having a hydrophobic portion in the surfacethereof was used. When the negative electrode having a hydrophobicportion in the surface thereof was used, the inner pressure in chargingthe battery was stabilized in spite of repetition ofcharging/discharging cycles compared with the case where a negativeelectrode having no hydrophobic portion was used. Further, in this case,the quantity of FEP to be added to the separator could be reduced.

From the double point of view of the battery inner pressure in chargingthe battery and the intermediate voltage in discharging the battery, itis preferable that the quantity of FEP to be added to the negativeelectrode is not larger than 1.5 mg/cm² as the mean value per unitnegative electrode area.

Having described the case where the invention is applied to anickel-hydrogen storage battery, it is a matter of course that theinvention is applicable to other alkaline storage batteries, such as amanganese dioxide-hydrogen storage battery, using a hydrogen absorbingalloy negative electrode.

As described above, according to the present invention, hydrophobicresin or hydrophobic material containing hydrogen absorbing alloy powderhaving a hydrogen equilibrium pressure lower than that of the hydrogenabsorbing alloy as a main constituent member of the electrode, anelectrically conductive material and a material capable of catalyzingthe decomposition of hydrogen gas is provided in the vicinity of thesurface of the hydrogen absorbing alloy negative electrode. As a result,an effect arises in that a closed alkaline storage battery free from theincrease of the battery inner Pressure in overcharging of the batteryand free from the decrease of the battery voltage in discharging thebattery can be provided.

We claim:
 1. An alkaline storage battery comprising: a positiveelectrode containing a metal oxide as a main constituent materialthereof, a negative electrode containing, as a main constituent materialthereof, a hydrogen absorbing alloy, an alkaline electrolytic solution,and a separator, in which said separator contains a hydrophobic materialdisposed in a part of the surface layer of said separator being incontact with said negative electrode, and said negative electrodecontains a material capable of catalyzing the decomposition of ahydrogen gas, in the inside of the negative electrode or in the surfaceof said negative electrode.
 2. An alkaline storage battery comprising: apositive electrode containing a metal oxide as a main constituentmaterial thereof, a negative electrode containing, as a main constituentmaterial thereof, a hydrogen absorbing alloy, an alkaline electrolyticsolution, and a separator, in which a hydrophobic material is containedin a part of the surface layer of said separator being in contact withsaid negative electrode, and a material capable of catalyzing thedecomposition of a hydrogen gas and an oxygen gas is contained in theinside of said battery in a state in which said material is free fromelectrical contact with said positive and negative electrodes.
 3. Analkaline storage battery according to claim 1, wherein said negativeelectrode has a hydrophobic portion in at least a part of the negativeelectrode.
 4. An alkaline storage battery according to claim 2, saidnegative electrode containing a hydrophobic portion in at lest a part ofthe surface layer of said separator being in contact with said negativeelectrode.
 5. An alkaline storage battery according to claim 1, whereinsaid hydrophobic material is disposed within greater than 50% of saidsurface layer of said separator.
 6. An alkaline storage batteryaccording to claim 22, wherein said hydrophobic material is disposedwithin greater than 50% of said surface layer of said separator.